I have an ASUS M3N78-VM motherboard with a 8200 integrated video card. I also use another computer with a ASUS Gf 210 with 512 MB video memory.
I use Debian Squeeze 32-bit on both computers. The cards work fine.

But the nvidia-settings GUI shows 512 MB video memory for the 8200 - But the system reports ~1800 MB free usable memory (from 2 GiB) and the cards specs go up to 256 MB shared.
For the Gf 210, nvidia-settings shows 1024 MB - i know that the card has 512 MB.
Nvclock doesnt know both cards, but reports 256 MB and 512 respectively (the Gf 210 is listed with 128-bit memory bus by nvclock that is incorrect, nvidia-settings reports 64 as it is).

These nvidia-settings reportings are like this since i got these cards, ranging from 185/190 series up to the 256 series drivers.
My question is why these readings are wrong ?

No, the cards work fine.
I am not keen on modifying stuff like this. I would like to know that TurboCache can cause instability/performance loss in cases when little memory is free.
I did google around for TurboCache, but it is advertised for the 6/7 series of cards - no mention of it on ASUS site for the Gf 210 or the 8200 IGP.
I have a few questions about this technology:

I wonder why does the 8200 use TC if it uses shared memory only?
When does TC start using additional memory?
I mean how the memory usage is prioritized - TC uses memory arbitrarily or only memory that is not used by the OS?
Can this memory be recovered by the OS if needed?
Can cause system instability if the system is using most of its memory and TC needs it?

All GPUs from the GeForce 8 series and up support TurboCache. For a discrete card, TC memory is slower than dedicated video memory, but faster than memory that the GPU can't access at all, which is what you got on non-TurboCache chips when you ran out of vidmem. For mGPUs, I'd expect TC memory and the dedicated memory to be roughly the same speed. However, the dedicated "carveout" memory is reserved for the GPU at boot time, while TC memory is dynamically allocated and shared with the OS. The OS cannot reclaim it while it's in use, but if you stop using it as TC memory then sure, the OS can reclaim it for other uses.

I don't think there are any known stability problems with TurboCache right now.